BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to network equipment deployed in the physical plant and, more particularly, to a method and apparatus for remotely interfacing a network device in the physical plant.
2. Description of the Related Art
Data communication networks may include various computers, servers, nodes, routers, switches, hubs, proxies, and other devices coupled to and configured to pass data to one another. These devices will be referred to herein as “network devices.” Data is communicated through the data communication network by passing data packets (or cells, frames, or segments) between the network devices by utilizing one or more communication links. A particular packet may be handled by multiple network devices and cross multiple communication links as it travels between its source and its destination over the network.
Many individual subscribers access the public communications network over twisted wire pair subscriber loops that terminate at a central office. Different levels of service may be provisioned over subscriber loops, such as standard telephone service or higher speed data services. One example of a high speed data service is referred to herein as Digital Subscriber Line (DSL) service. There are many types of DSL service that may be utilized to communicate between the central office and subscribers and the invention discussed herein is not limited to any particular type of DSL service.
Conventionally, electrical equipment has been deployed at the central office in a controlled environment to interface with subscribers over subscriber loops. For example, a DSL Access Multiplexer (DSLAM) may be used to interface between multiple DSL-provisioned subscriber loops and the high bandwidth transmission capabilities on the backbone of the communications network. Typically, only relatively simple electrical equipment, such as signal repeaters, have been deployed outside the central office between the central office and the subscribers. The area outside of the environmentally controlled central office will be referred to herein as the “physical plant.” Equipment deployed in the physical plant may be deployed in areas such as on a telephone pole, in a manhole, curbside on a platform, or on aerial wires between telephone poles.
Technical limitations associated with DSL may prevent a service provider from providing DSL service to a subscriber residing more than a predetermined distance from the central office. Additionally, the bandwidth available to a DSL subscriber is generally related to the length of the subscriber loop over which DSL service is to be provided. One proposal to enable larger numbers of customers to subscribe to higher bandwidth DSL services is to deploy active electronic equipment, such as a DSLAM, in the physical plant. Deploying a DSLAM in the physical plant enables the distance between the subscriber and the DSLAM to be reduced, thus increasing the number of customers that may be provisioned with DSL services and increasing the bandwidth that may be provided to existing DSL subscribers.
Deploying active equipment in the physical plant, however, requires the equipment to be maintained and upgraded in the physical plant. Often maintenance and repairs may be effected through the network remotely. However, in particular instances it is advisable or necessary to send a technician out into the physical plant to service the network device. Unfortunately,. device placement and weather conditions may make it difficult or uncomfortable to access the network device in the physical plant.
Where there is a problem on the optical link connecting the active equipment in the physical plant and the central office, it is also desirable to isolate the problem to be able to dispatch a technician to the proper location outside the central office. It would be advantageous, in these situations, to enable the active equipment in the physical plant to participate in evaluation of the fiber. Additionally, it would be advantageous, in these circumstances, to enable emergency services to be provided until the problem in the optical link is able to be resolved.
SUMMARY OF THE INVENTIONThe present invention overcomes these and other drawbacks by providing a method and apparatus for remotely interfacing a network device in the physical plant. Remotely interfacing the network device allows a network technician to monitor, interrogate, and control, the network device, and upload new software to the network device without requiring the technician to be physically resident in the network device's environment. Remotely interfacing the network device may take place over the optical facilities connecting the network device to the central office, over existing copper facilities configured to provide power to the network device, or over one or more wireless links.
According to one embodiment of the invention, the network device deployed in the physical plant includes a wireless transceiver to enable the technician to interface with the network device over a wireless control channel. Providing a wireless interface to the network device allows the technician to access control and maintenance information on the network device such as the network device's management information base, monitor the network device's operation, interrogate the network device to ascertain specific aspects of its operation or aspects of the network on which it is operating, and control the network device to cause the network device to perform particular desired operations. Examples of operations include performing network device reset, interacting with other network devices on the network, and numerous other control and diagnostic functions. Additionally, providing a wireless interface enables the network technician to fix specific perceived problems in the network device, such as by uploading new software to the network device to enable the network device to have increased functionality or more reliable performance.
The network device may also be configured to enable communications to take place over existing copper facilities configured to provide power to the network device or through a wireless control unit available to the network device over the wireless interface. These communications enable the network device to provide emergency services, such as 911 services, as well as enable the network device to participate in testing aspects of the network on which it is operating, such as by participating in Optical Time Domain Reflectometry (OTDR) testing of the optical fibers connected to the network device.
The wireless link may be formed via any convenient technology, such as one of the IEEE (Institute of Electrical and Electronics Engineers) 802.11 technologies, other wireless technology, Infrared Data Association (IrDA)-based technology or other infra-red based technology, or ultrasonic-based technologies. Infra-red and ultrasonic transmissions may be advantageous in certain applications, such as where the network device is submerged in a manhole filled with water or other fluids. 802.11-based technologies and other wireless technologies are advantageous in certain applications as they do not rely on line-of sight communications and thus may allow the technician greater flexibility in selecting a convenient site from which to interface with the network device.
According to another embodiment of the invention, the wireless transceiver may be accessed by wireless subscribers when not being used by the technician to interface with the network device. Where the wireless technology used to interface with the network device is based on 802.11 or another commonly used wireless networking technology, it is possible to enable the network device to act as an 802.11 hot spot or other network access point thus allowing for the convenient deployment of wireless network access points in the physical plant. The network device deployed in the physical plant may be configured to receive power from the central office over existing copper facilities such as one or more existing subscriber loops. According to an embodiment of the invention, the existing copper facilities may be utilized to allow the network device to be accessed remotely from the central office to enable the central office to perform various maintenance and control operations on the network device, such as to allow the network device to participate in-physical layer testing of the optical fiber connecting the central office and the network device and to enable the continuation of emergency services during the network outage.
BRIEF DESCRIPTION OF THE DRAWINGSAspects of the present invention are pointed out with particularity in the appended claims. The present invention is illustrated by way of example in the following drawings in which like references indicate similar elements. The following drawings disclose various embodiments of the present invention for purposes of illustration only and are not intended to limit the scope of the invention. For purposes of clarity, not every component may be labeled in every figure. In the figures:
FIG. 1 is a functional block diagram of a network architecture according to an embodiment of the invention;
FIGS. 2-5 are functional block diagrams of placement of network devices in the physical plant according to embodiments of the invention;
FIG. 6 is a functional-block diagram of an Optical Network Unit-Digital Subscriber Line Access Multiplexer (ONM) equipped with wireless transceiver ports according to an embodiment of the invention;
FIG. 7 is a functional block diagram, of a network device equipped with wireless transceiver circuitry according to an embodiment of the invention;
FIG. 8 is a network diagram of a network device capable of being interrogated by a control unit and accessed by subscribers over a wireless network; and
FIG. 9 is a functional block diagram of a control unit according to an embodiment of the invention.
DETAILED DESCRIPTIONThe following detailed description sets forth numerous specific details to provide a thorough understanding of the invention. However, those skilled in the art will appreciate that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, protocols, algorithms, and circuits have not been described in detail so as not to obscure the invention.
As described in greater detail below, according to an embodiment of the invention, a network device deployed in the physical plant includes a wireless access port to enable the network device to be accessed by a control unit. Enabling the network device to be accessed by a control unit enables a technician servicing the network device to monitor or perform functions on the network device remotely. Thus, where the network device is suspended above the ground, the network technician may stay safely on the ground while performing device and network diagnostics, upgrades, or other maintenance and management functions on the network device. Similarly, where the network device is deployed in a manhole or other underground enclosure, the network technician does not need to empty the manhole (if filled with water or other fluid or gas) to access the network device, and may stay above-ground and out of traffic while remotely interfacing with the network device. The wireless access may also assist with inventory management in the physical plant by allowing the network devices to be interrogated as to identity. Additionally, prior to physically accessing the network device, the wireless access may enable emergency services to be provide and enable the network device to communicate problem identification information so that an appropriate technical team may be dispatched to interface with the network device. Any number of functions may be performed while interfacing the network device, and the invention is not limited to any particular subset of possible functions or operations.
According to another embodiment of the invention, remote access to the network device deployed in the physical plant may occur over existing copper facilities configured to provide power to the network device. In this manner, communication with the network device may be maintained even when the optical facilities have been compromised. This allows the network device to participate in physical layer testing of the optical facilities and may provide a limited amount of connectivity between the central office and subscribers interfacing the network device, such as to provide emergency services, e.g. 911 services.
FIG. 1 illustrates a network architecture in which active electronic network equipment is deployed outside the central office in the physical plant. As illustrated inFIG. 1, acentral office10 provides connectivity betweensubscribers12 and acommunications network14. The communications network in one embodiment may be the Internet or another data centric communications network. Activeelectronic equipment16 is deployed outside of the central office, in the physical plant, and interposed between thecentral office10 and thesubscribers12.
In one embodiment, a passiveoptical network18 may be utilized to connect an Optical Line Terminating (OLT) network device at thecentral office10 with an Optical Network Unit (ONU) integrated into the activeelectronic equipment16. In the embodiment illustrated inFIG. 1, the activeelectronic equipment16 also includes a Digital Subscriber Line Access Multiplexer (DSLAM), to enable the activeelectronic equipment16 to aggregate DSL signals from DSL subscribers and pass the DSL traffic back to the central office. In the following description, the activeelectronic equipment16 incorporating both an ONU and a DSLAM will be referred to as an Optical Networking Multiplexer (ONM). The invention is not limited to this embodiment but rather extends to all embodiments wherein active electronic equipment is deployed in the physical plant.
DSL enables high-bandwidth services to be provisioned over existing twisted pair copper telephone wires commonly referred to as “subscriber loops.” Several different types of DSL technology have been developed, including Asymmetrical DSL (ADSL), High-bit-rate DSL (HDSL), Rate Adaptive DSL (RDSL), Symmetric DSL (SDSL), and Very high speed DSL (VDSL). These, and other DSL-based technologies, will be referred to herein collectively as xDSL. Subscribers may communicate with the DSLAM over anetwork20 using any xDSL technology, and the invention is not limited to any particular xDSL technology.
The activeelectronic equipment16, such as the ONM illustrated in the figures, may be placed in many different locations in the physical plant.FIGS. 2-5 illustrate several possible places where active electronic equipment may be deployed in the physical plant. As shown inFIGS. 2-5, the ONM may be deployed in a manhole (FIG. 2) on a pedestal for example along side a junction wire interface (JWI) (FIG. 3), on a telephone pole (FIG. 4) or suspended on wires between telephone poles (FIG. 5). The ONM may be deployed in other locations as well, such as on the side of a building, or buried underground, and the invention is not limited to only these particular locations illustrated inFIGS. 2-5.
The network device may be housed in a reentrant packaging such as the packaging currently utilized to house splice enclosures. Utilizing a familiar reentrant packaging enables the ONM to utilize the same stack up and installation techniques that are currently used in connection with passive splice boxes. One example of a possible reentrant package usable in connection with the ONM according to an embodiment of the invention is the Armadillo™ brand enclosure currently used for fiber splices and available from Preformed Line Products. The invention is not limited to an implementation utilizing this particular packaging, however.
In one embodiment, optical fibers configured to be attached to optical facilities and copper wires suitable for splicing to subscriber loops and power lines are pre-attached to the device before the network device is shipped out to be installed in the physical plant. This facilitates deployment and allows the network devices to be installed in the physical plant by technicians not skilled in sealing reentrant packages or in commissioning the equipment. Splice boxes may be used to splice the pre-attached copper wires and optical fibers to subscriber loops and optical cables running from the central office, and to subscriber loops extending toward xDSL subscribers.
FIG. 6 illustrates an embodiment of a network device containingONM circuitry22 configured to enable the ONM to communicate withxDSL subscribers12, thecentral office10, and a control unit (described below). As shown inFIG. 6, theactive ONM circuitry22 includesONU circuitry24 configured to communicate over passiveoptical network18 withcentral office10, andDSLAM circuitry26 configured to communicate overxDSL network20 withsubscribers12. Wireless circuitry28 is provided to enable theONU16 to be interfaced via the control unit. Optionally, powerline communication circuitry29 may be provided to enable the ONM to communicate with the central office over the subscriber loops and other existing copper facilities configured to provide power to the ONM.
One or moreoptical ports30 andelectrical ports32 are provided to interface with the physical transmission media forming the optical and electrical networks in which the ONM will operate. Apower port33 may be provided to enable power to be supplied to the ONM and to enable powerline communication to take place with the central office. The ONM may be configured to communicate utilizing different protocols with the central office than with the DSL subscribers. According to one embodiment, the ONM communicates with the central office utilizing Ethernet over ATM, although the invention is not limited to this embodiment as any combination of protocols may be used to exchange data between the ONM and the central office.
TheONM device16 also one or morewireless ports35, such as anRF port34, anIR port36, and anultrasonic port37. The RF port may include an internal antenna configured to enable wireless access to thenetwork device16. Alternatively, the internal antenna may be replaced or supplemented through the provision of a port through which theONM16 may be connected to an external antenna. The InfraRed (IR)port36 may be provided to enable the network device to communicate with the control unit utilizing infrared signals. TheIR port36 may be configured to communicate using the Infrared Data Association (IrDA) standard or another standard or proprietary protocol for enabling IR transmission of packets. Optionally, anultrasonic port37 may be included to enable theONM device16 to be accessed via one or more ultrasonic communications technologies.
The wireless circuitry28 is configured to process wireless signals and implement instructions received over thewireless ports35 on the ONM. The invention is not limited to a network device employing two or more types of wireless access, as it may be desirable under certain conditions and for particular applications to only utilize one type of wireless access. Similarly, it may be desirable under particular circumstances to enable more than three different technologies to access the network device. Accordingly, the invention is not limited to the illustrated embodiment employing IR, ultrasound, and wireless remote access, but rather extends to all manners of obtaining remote access and all combinations of technologies. The example illustrated inFIG. 6 is merely an example of one manner of configuring the network device that will be used for purposes of explaining the invention. The invention is not limited to this particular example.
Any wireless technology may be utilized to interface with the network device. For example, transmissions may take place in one or more of the unlicensed spectrum bands (902-928 MHz, 2.4-2.4835 GHz, or 5.725-5.850 GHz). Transmissions may also take place in one or more licensed spectrum bands. The transmissions may be formed utilizing one or more wireless standards. Examples of wireless standards include Wi-Fi (802.11b), Bluetooth, HIPERLAN/2, 802.11a, 802.11g, technologies based on other 802.11 standards, HomeRF (Home Radio Frequency) and DECT (Digital Enhanced Cordless Telecommunications) as well as competing technologies such as GPRS/EDGE (General Packet Radio Service/Enhanced Data rates for Global Evolution), 3G (third Generation) mobile and TETRA (Terrestrial Trunked Radio). The invention is thus not limited to a particular frequency or wireless transmission protocol.
According to one embodiment of the invention, the wireless technology selected to enable the network device to be interfaced remotely is based on a technology that is commonly used by wireless network subscribers. For example, the wireless channel may be implemented in the UMTS licensed spectrum, spectrum utilized by other cellular technology-based standards, or spectrum utilized in implementation of on one of the 802.11 standards. As discussed in greater detail below, this allows the network device to provide wireless network access to subscribers as well as enabling the network device to be interfaced by a technician.
FIG. 7 illustrates the network device ofFIG. 6 in greater detail. As shown inFIG. 7, anONM16 according to an embodiment of the invention includesoptical ports30 configured to connect with optical fibers linking the ONM with thecentral office10. Although the ONM according to the invention includes optical ports facing the central office, the invention is not limited to this embodiment as other ports may be utilized as well, such as electrical ports, point-point infrared, wireless, or other ports configured to transfer data over another convenient transmission medium.
In the embodiment ofFIG. 7, the optical ports are connected to optical-electrical/electrical-optical conversion circuitry38. O-E/E-O conversion circuitry38 is configured to convert optical signals received overoptical ports30 into electrical signals for processing by the other circuitry ofONM16. The E-O portion of O-E/E-0 circuitry is configured to convert electrical signals into optical signals to be transmitted to the central office over theoptical ports30. Where communications between theONM16 andcentral office10 are based on an electrical physical layer, theconversion circuitry38 may be eliminated.
The ONM contains aprocessor40 havingcontrol logic42 configured to enable it to controlONM16 and otherwise perform functions ascribed to an ONU and a DSLAM.DSLAM code44,ONU code46, and other data and/or instructions are stored inmemory48 for use by thecontrol logic42 to enable it to perform the functions required of it to participate in communicating with network administrators, users, and other network devices. Interactions on the network and during protocol exchanges with other network devices on the network may be facilitated through the implementation of aprotocol stack50 containing instructions and data relevant to communications protocols commonly used on the network and by the network devices.
Thecontrol logic42 may be implemented as a set of program instructions that are stored in a computer readable memory within the network device and executed on a microprocessor within the network device. However, it will be apparent to a skilled artisan that all logic described herein can be embodied using discrete components, integrated circuitry, programmable logic used in conjunction with a programmable logic device such as a Field Programmable Gate Array (FPGA) or microprocessor, or any other device including any combination thereof. Programmable logic can be fixed temporarily or permanently in a tangible medium such as a read-only memory chip, a computer memory, a disk, or other storage medium. Programmable logic can also be fixed in a computer data signal embodied in a carrier wave, allowing the programmable logic to be transmitted over an interface such as a computer bus or communication network. All such embodiments are intended to fall within the scope of the present invention.
Apacket queue52 is implemented to store packets received for transmission over theoptical network18 or over theDSL network20. The packet queue may store packets for transmission where, for example, data is received faster than it can be processed or output on the network. Aswitch fabric54 enables packets to be directed to theproper output port32 when more than one output electrical port is implemented on theONM16. Amultiplexer56 receives packets in parallel on thevarious DSL ports32, serializes the data, and passes the data to the switch fabric. Optionally, themultiplexer56, alone or in combination with thepacket queue52, may apply policies to incoming and outgoing traffic and otherwise enforce service level agreements with DSL subscribers.
Acontrol module58 enables a control unit to interface the ONM viaRF port34, infra-red port36,ultrasonic port37,other wireless ports35, orpowerline data port33. The control module may enable multiple functions of the ONM to be interrogated, modified, and/or controlled. For example, new or modified DSLAM code may be updated to the ONM via thecontrol module58. In another embodiment, diagnostic programs may be downloaded and run on the control module or run on the handheld control unit to enable the ONM to be interrogated to ascertain a likely source of malfunction, perform reset operations, or perform any other conventional functions onONM16. Additionally, network verification tests may be run via the control module to cause the processor to output signals on the networks to which it is connected to determine a likely cause and/or location of network failure. The invention is not limited to any particular set of functions performable by interfacing theONM16.
The ONM may receive power from existing copper facilities deployed in the physical plant, such as one or more subscriber loops connected between the central office and the ONM, and may also obtain limited communication services from the central office over the existing copper facilities. According to one embodiment of the invention, the subscriber loops may be utilized to remotely access the ONM to interrogate, provision, and control the ONM, and to enable the ONM to participate in physical layer testing of the optical facilities interconnecting the central office and the ONM. For example, the ONM may be configured to perform Optical Time Domain Reflectometry (OTDR) tests on the optical fibers to which it is connected to help locate a likely location on the optical fiber where a break or other fault might have occurred. Additionally, emergency services, such as 911 services, may be provided over the existing copper facilities. Information associated with the OTDR tests and emergency services may be relayed back to the central office over the existing copper facilities utilized to provide power to the ONM using a conventional transmission standard, such as a telephone modem standard or an xDSL standard. The invention is not limited to any particular manner of communicating over the existing copper facilities with the CO. Where the ONM is not provided with intelligence, e.g. software, instructions and data sufficient to enable it to participate with the CO may be received by the ONM over the existing copper facilities or from the wireless control unit.
As described in greater detail below in connection withFIG. 8, one or more of thewireless ports35 may be configured, according to one embodiment of the invention, to allow wireless subscribers to access thenetwork14 through theONM16. In this embodiment, data packets received over thewireless port35 are passed to thepacket queue52, theswitch fabric54, or other constructs inONM16 configured to receive and/or process packets.
FIG. 8 illustrates aONM16 having one or morewireless access ports35 according to an embodiment of the invention. In this embodiment, the wireless port is accessible bywireless subscribers60 in addition to awireless control unit62 having awireless transceiver64.Wireless control unit62 is described in greater detail below in connection withFIG. 9. As shown inFIG. 8, multiple wireless subscribers, such as 802.11wireless subscribers60 or cellular wireless subscribers, may access the network throughONM16 to thereby enable theONM16 to function as an 802.11 hotspot or a cellular base transceiver station. The invention is not limited to an 802.11-based wireless communication protocol, as described above.
FIG. 9 illustrates acontrol unit62 configured to interface with theONM16 described above. As shown inFIG. 9, thecontrol unit62 includes one or morewireless ports64, such as aninfrared port66, anultrasonic port67, and anRF port68. These ports enable the control unit to communicate with theONM16 to perform various control, maintenance, and other functions, and to provide emergency communication services, as described above. Optionally, an extension may be provided to enable an optical probe, for example connected to the end of a telescoping pole, to be connected to thecontrol unit62. Attachment of an optical probe is useful, for example, where theONM16 is deployed in a manhole and submerged. Placing the optical probe below the waterline eliminates surface reflections and thus enables stronger IR signals to be exchanged between thecontrol unit62 andONM16. Additionally, inclusion of an IR-based communication port or other communication port capable of operating through water enables communications to take place with the network device without requiring the manhole to be emptied of water and other fluids and gasses such as natural gas. Utilization of ultrasonic communication may be advantageous where the IR port is compromised or obscured by dust or other debris. Also, communication may take place without requiring the technician to enter an unknown environment, such as a manhole, which may be occupied by insects and other creatures.
Thecontrol unit62 contains aprocessor70 havingcontrol logic72 configured to enable it to controlONM16 and otherwise perform functions required of a conventional hand-held control unit.DSLAM code74 andONU code76 may be stored oncontrol unit62 so that it may be uploaded onto ONMs deployed in the physical plant. Other data and/or instructions are stored inmemory78. Interactions with theONM16 may be facilitated through the implementation of aprotocol stack80 containing instructions and data relevant to communications protocols commonly used on the network and by the network devices.
According to one embodiment of the invention, the control unit is an intelligent control unit having a processor and software configured to interface with a relatively unintelligent ONM. The ONM, in this embodiment, includes sufficient hardware to enable it to perform instructions passed to it from the control unit, but does not include diagnostic software or other software to perform self-diagnostic functions or self-repair functions. In this embodiment, the intelligence is maintained in the control unit to enable the control unit to interrogate and reconfigure itself. Upon input of instructions from the control unit, the hardware on the ONM will perform specified diagnostic and configuration processes, to enable it to analyze what is happening on the ONM and reconfigure itself through the assistance of the control unit. Examples of diagnostic and self repair that may take place include ascertaining a malfunction on a particular node and trying a different node, ascertaining a problem in the ONM power supply and attempting to attach to an alternative power supply. Numerous other diagnostic and self repair functions may be performed as described above and the invention is not limited to this select few described functions.
An interrogation andcontrol module82 interfaces between the processor and the data ports to facilitate interrogation and control functions. Adisplay84 controlled by adisplay controller86 provides information to the user about the status of the ONM. An user input88 allows the user to input instructions to thecontrol unit62 so that the user can specify actions to be taken on the ONM. The user input may be a key pad, key board, stylus, mouse, track pad, track ball, soft keys or any other type of user input device configured to enable the user to input information into the control unit or to otherwise control the operation of the control unit. The control unit may also include an access port to enable it to be interfaced by a computer system. One or more of thewireless ports64 may also be utilized for this purpose.
Optionally, a physical and/orlogical security module90 may be included to prevent unauthorized persons from operating the control unit. In one embodiment, the security module is a physical module that requires the operator to possess a particular object to access the device, such as a key, or an access card. In another embodiment, the security module is a logical security module, implemented in software, firmware, or hardware, that requires the user to input a passcode or other identification information prior to enabling the user to access the device. In another embodiment, both physical and logical security modules are implemented. The invention is not limited to any particular implementation of the security aspect of the hand-held control unit.
TheRF port68, according to one embodiment of the invention, is also configured to communicate via standard wireless communications protocols, over a cellular or PCS network, to enable the control unit to communicate with a central network prior to, during, or after communicating with theONM16. The ability to interface between theONM16 andcentral office10 over a wireless link enables the ONM to communicate with the central office even where the link between the ONM and the central office has been compromised. This enables the control unit to provide emergency communication services to theONM16 where, for example, the optical link to the central office has failed.
Optionally, as described above, an optical extension, such as an optical extension attached to a telescoping or other pole may be attached to the control unit to enable the IR port to be placed in closer proximity to the network device.
It should be understood that various changes and modifications of the embodiments shown in the drawings and described in the specification may be made within the spirit and scope of the present invention. Accordingly, it is intended that all matter contained in the above description and shown in the accompanying drawings be interpreted in an illustrative and not in a limiting sense. The invention is limited only as defined in the following claims and the equivalents thereto.